A cable head used in petroleum logging is a connection apparatus between a logging cable and a logging instrument. A conventional cable head for petroleum logging includes a coupling sleeve, an upper joint, a lower joint and a housing. A tension rod is connected between the two joints. The coupling sleeve is connected with the upper joint, and the housing is connected with the lower joint. In use, the logging cable and the logging instrument are connected with the coupling sleeve and the lower joint respectively, and the cable head is filled with silicone grease, which causes a lot of trouble in assembly, disassembly and maintenance.
In view of this, Chinese patent CN 2641287 discloses an electrode, cable logging cable head including a main joint and an inner torpedo, in which two ends of the tension rod are fixed between the main joint and the inner torpedo respectively, and a fully-sealed measure is employed between the connecting parts. Thus, there is no need to fill the silicone grease in the cable head in use.
In logging, the cable head is sometimes stuck in the well due to failure in operation. At this moment, the logging cable is needed to be disconnected from the cable head for recovering the logging cable, and the current method is to break the tension rod in the cable head by pulling the logging cable. It is well known that, the breaking force of the tension rod is designed based on the safe tension force which is applied to the logging cable. In order to break the tension rod, the cable of the above cable head have to first overcome the forces applied on the joints of the cable head by various fluids (for example oil, water or mud, etc) in the well. However, since the operation failure often happens at one or several kilometers depth in the well, the force applied on the cable head by various fluids in the well is very great. For example, at the depth of 1 km to 1.5 km in the well, the force applied on the upper joint by the fluid is approximately equal to the breaking force of the tension rod, and in this case, the actual tension force of the logging cable is required to be twice as much as the set tension force to break the tension rod. In particular, as the depth of the well increases, the actual tension force of the cable would increase accordingly.
Apparently, when the logging cable head is stuck in the well, the logging cable head with the conventional structure has disadvantages such as damage or even discard of the logging cable.